In the production of cast parts made of aluminum alloys it has always been thought in the past to be necessary for many such castings (especially with a T6 or T7 temper) to undergo an elaborate heat-treating process in order to impart to the cast parts the necessary mechanical properties (like hardness and tensile strength required for the demanding working uses of said parts).
It is known that the degree of hardness and other mechanical properties of the cast parts depend on the thermal history of the cast parts after having been cast in the mold. The Aluminum Association (AA) has classified the most used aluminum alloys and the several standard heat treatments used in the industry. Examples of such standard heat-treatments those denominated T6 and T7, which designate a standard set of mechanical properties developed by certain castings of primarily silicon-copper-aluminum alloys.
The automotive industry throughout the world demands very strict quality standards. Casting plants making aluminum motor parts must therefore be able to produce cast parts which consistently comply with the minimum levels of mechanical properties specified for each part. Since quality is a must, the casting plants follow those procedures and processes which are well tested and have proven reliable for many years. The production process currently followed in the industry comprises filling a mold with liquid aluminum alloy, cooling the cast part in the mold in order to obtain a solidified casting, extracting the casting from the mold, and allowing the cast part to cool-down naturally to ambient temperatures, and then subjecting batches of such cooled castings to the aforementioned "solution" heat-treating process. One way to reduce the heat load in the solution heat treatment: furnace, has been to remove the sand cores and riser portions of the castings after natural cooling and before the "solution" heat treatment. The heat treating of the prior art comprises heating the preferably trimmed castings in a furnace to temperatures above about 470.degree. C. (typically in the range between 480.degree. C. and 495.degree. C.) for a certain period of time, usually in the range between at least 2 to 7 hours. This treatment is performed in order to bring back into solid solution the copper and/or other alloying elements that give the castings their hardness. It is known that, while the casting metal is in the molten state, the alloying elements are in solution in the aluminum substrate. During the cooling process, particularly if the cooling is carried out at a slow rate, there is a tendency for the different elements to become segregated. Therefore, traditionally the casting is re-heated in a "solution" heat treatment furnace for several hours, and only then is quenched, i.e. rapidly cooled down by a fluid quench from a temperature for example about 480.degree. C. to around 85.degree. C., so that the solid solution is preserved (before segregation can occur). Such post solution-treatment quench cooling may commonly be continued in a manner sufficient to bring the castings down to any of a number of different temperatures and at different rates according to the final properties of the alloy to be emphasized.
This quenching step produces a supersaturated solid solution that causes the alloy to harden naturally as time passes. Finally, in order to accelerate and improve this age hardening, the quenched castings are maintained at temperatures of about 200.degree. C. in an "aging" furnace for about 2 or more hours. The time spent in the "Eaging" furnace at elevated artificial aging temperatures brings the alloy to at least a partial coherency in its structure giving the required hardness and strength properties.
U.S. pat. No. 5,788,784 to Koppenhoefer et al. discloses a process for heat-treating light metal castings that requires "a solution heat treatment furnace 2, an adjoining quenching device 3, as well as an aging furnace 4", all particularly for cylinder heads of piston engines. In the U.S. Pat. No. 5,788,784 process, after solidifying and removing the casting from the mold, said castings unconventionally are not naturally cooled, yet are still solution heat treated (claiming the advantage of using the residual heat of the casting present at the approximate 530.degree. C temperature of such treatment). Thereafter, the castings are quenched with an air/water mixture down to 130.degree. C. to 160.degree. C., and then aged in a furnace at approximately 170.degree. C. to 210.degree. C. (thus taking advantage of some relatively minor residual heat carryover into the aging furnace), and are then finally cooled to room temperature after, for example, four hours of furnace aging. The castings are individually quenched with a mist-type fine mixture of air and water, which is "nozzle sprayed on all sides" of the casting.
Koppenhoefer asserts a number of advantages by reason of quenching the castings with an air-water spray, for example that a uniform and low-distortion cooling is achieved, that the adhering core sand is not wetted at the elevated quenching temperatures and can be collected clean and reused after regeneration, and that the residual heat of the casting remaining at 130.degree. to 160.degree. C. can be used to aid in the subsequent furnace aging step (by not cooling down the casting too much and leaving some heat in said casting). Quenching the casting by directing the sprayed water on all sides of the casting suggests that most of the residual heat is lost, with that amount retained being mainly in the inner portion of the casting. This also suggests that a large temperature gradient would have to be maintained between the interior and the surface of the casting in order for the amount of retained residual heat carried over into the aging step to be meaningful. Such large differentials in temperature across the casting (particularly the end product portion thereof) is one of the problems to be avoided while quenching a piece in order to avoid stresses and achieve the T6 or T7 properties and also to avoid spheroidization of the alloying elements. Furthermore, Koppenhoefer does not teach or suggest applicant' invention of selectively quenching only the end product portion of the casting in order to use eventually the unquenched retained residual heat from the sprue and from any other temporarily retained waste portion of the casting (including sand cores) in order to enable aging of said casting without need for an aging furnace. In contrast, Koppenhoefer teaches decoring the resin bonded sand cores from the castings by being "pyrolytically destroyed" during solution heat treatment and further removed during quenching, all prior to aging.
U.S. Pat. No. 5,112,412 to Plata et al. teaches a process for cooling large cast billets of aluminum after a temperature homogenization (re-heat) annealing step. Annealing is a softening process for aluminum (just the opposite of the strengthening and hardening heat treatment of the present invention), and this Plata patent is silent on how the cooling is to be done to accomplish a particular result (mainly mentioning only that it be "in accordance with the alloy composition" and describing how the "automated and controlled manner of spraying can be adjusted to different shaped billets, as they may differ from the usual round shape"). This patent first describes cooling the annealed billet with a spray on all sides. This decreases the temperature at the surface of said billet, while the center portion (inaccessible to the spray) necessarily cools more slowly and thus initially remains at a relatively higher temperature. The billet leaves the spray and is allowed to equalize its internal and external temperatures in an insulated chamber. In another embodiment, Plata et al describes a process modification in the case of a so-called (but otherwise unidentified) "hard" alloy to continue spraying until the billet has achieved an equalized temperature. An example of this temperature is giver as "310 C.-350 C. in AlMgSi alloys" (a range above most age hardening but typical of softening annealing). The teaching includes the possibility of varying the intensity of the continuous spray, but only for the purpose of achieving a "better balanced heat flow" and a temperature zone "preferably distributed homogeneously during cooling so that no or only minimal deformations, stresses or cracks form". For example, the patent states that circular billets are sprayed evenly, but a rectangular billet may be sprayed with different intensity along it periphery. This difference in spray intensity is to achieve uniformity of cooling during the quench (just the opposite of subjecting the casting to a significant differential or complete absence of quench cooling of a specific waste portion of the casting in order to maintain such portion at a significantly higher temperature during the quench of the work portion (and much less to identify such a waste portion which is accessible to the spray, but is not to be so spray cooled). Thus, even though one of the embodiments discloses a spray process involving a difference in the temperature between certain portions of the billet which later reach an equalized temperature, there is no disclosure of differential quenching of selected portions of the casting to promote rather than minimize an initial significant heat differential between selected different portions of the casting (particularly with the division being between equally exposed waste and workpiece portions). Furthermore, Plata et al. teaches a process of cooling the surfaces of the workpiece (billet) on all sides, while the inner portion of the workpiece remains hot. If this process is applied to the workpiece portion of the castings for cylinder heads or blocks for engines, it will cause a different distribution of the alloying elements and thus it will fail to achieve the objects of the present invention (which provides a quenching step to produce uniform properties such as those obtained with a T6 treatment, all with accelerated aging but without the need for an aging furnace). In applicant' casting, the unquenched portion is an existing waste portion that is put to a useful interim purpose but whose ultimate alloy and physical properties are irrelevant. Engine castings, if made by the Plata process would be rejected.
Applicant' recent U.S. Pat. No. 5,922,147 (to Valtierra et al.), mentioned above, discloses an improved heat-treating method whereby the castings are quenched immediately after having been extracted from the mold, thus eliminating "solution" heat treatment and avoiding the need for a solution heat treatment furnace; while nevertheless producing castings with similar properties to those that undergo the traditional solution heating step. The U.S. Pat. No. 5,922,147 patent process provides a casting plant with greatly improved productivity and significant savings in capital and operational costs. This patent, however, does not teach or suggest a method capable of eliminating also the aging furnace.